Ignition sources of explosion
Besides explosive atmosphere made of air (oxygen) and flammable matter in the for of dust, gas or mist, also ignition source is necessary for the explosion. It is the third apex of the explosion triangle (combustible matter, oxygen, ignition source). Ignition source is, generally said, such effect, which produces energy to its surroundings and this energy is sufficient to ignite the explosive atmosphere in which this source occurs. Energy necessary for the ignition is depending on the type of the explosive atmosphere and characteristics of the dust or gas. Generally can be said, that for ignition of gaseous atmosphere only 10-1000 times lower energy is sufficient, comparing to dust atmosphere.
Despite of the fact, that there can be unlimited reasons for forming the ignition source (and telling them after the massive explosion is the most complicated part of the investigation), there is only limited number of ignition sources and many of them is even not sufficient for the ignition of majority of the matters. The complete list of the ignition sources with the information if the source is or is not relevant in normal conditions for gases or dust is in the following table.
List of ignition souces:
| ignition source | common reason | relevant for dusts | relevant for gases and vapours |
| hot surface | friction of the rotating parts, failures of the devices working with high temperature, damages of heat insulations, processing of the materials, failures of brakes, barings, rotating mechanisms | YES | YES |
| flames, glowing particles, sparks | furnaces, burning chambers, devices for drying, devices working with open fire,... | YES | YES |
| mechanical sparks | friction, impacts, abrassive processes (grinding, polishing), foreign objects in the technology, cutting, breakes, bearing, rotating mechanisms,... | YES | YES |
| electric devices (discharge sparks, hot surfaces) | switching on and off the electric circuits, releasing of the contacts and connections, electric arc and short connections, heating, damaged lightings, bulbs,... | YES | YES |
| stray electric current | reverse current, damaged insulations of high voltage cables and devices, failures in electric installations, magnetic induction, back current circuit failures,... | rarely (depending on the type of the dust and energy of the ignition source) | probably yes |
| static energy | failures of grounding, fast separation processes, friction | rarely (only the most sensitive dusts) | almost always |
| lightning | lightning itself, heating of the lightning conductor, static energy induction in the close surrounding of the lightning | YES | YES |
| high-frequency electromagnetic waves | TV and radio broadcasting transmitter failurs, measuring, army and medical devices failure, high frequency generators (drying, hardening, welding, cutting),... | rarely | sometimes (depending on the energy of the source) |
| electromagnetic waves | concentrated rays and waves (lenses, reflectors), lasers (including measuring devices), strong sources of radiation,... | yes, but depending on the energy | YES |
| ionizing radiation | X-rays, radioactive materials, chemical reaction caused by radiation (disruptive exotermic reactions), microwaves,... | yes, but depending on the energy | yes, but depending on the energy |
| ultrasound | absorption of ultrasound waves | depending on energy, but in normal conditions unrealistic | rarely |
| adiabatic compression and shock waves | heat produced during fast and strong adiabatic compressions, heat caused by pressure shock wave going through the pipeline | no, unrealistic in normal conditions | rarely |
| exotermic reactions (including autogenous ignition) | chemical reactions, flammable matters, biochemical processes, polymeration reactions,... | YES | YES |
Important notice!
This table should never be understood as definitive and for all kinds of explosive atmospheres and conditions, because minimum ignition energy may vary in great range. Also these ignition sources can reach various energies depending on their origin (for example electromagnetic waves). If there is "rarely" or "depending on the conditions" in the table, it means during normal circumstances and in commonly used devices. Always keep in mind that theoretically any of these ignition sources may ignite any type of explosive atmosphere, during certain circumstances, so no ignition source can be fully ignored if there is at least theoretical chance of its appearance.